1. Field of the Invention
The present invention relates to a method for testing the domain switching induced in ferroelectric ceramics upon light irradiation, with the aid of an acoustic emission technique. Since photoinduced domain switching causes a change in the remanent polarization and strain of ferroelectric ceramics, the test thereof is very important in controlling optical properties such as photovoltaic currents and photostrains.
2. Description of the Prior Art
Being defined as the phenomenon of transient elastic-wave generation due to a rapid release of strain energy caused by a certain event, such as a structural alteration, in a solid material, acoustic emission (hereinafter referred to as "AE") is useful to non-destructively analyze the dynamic action generated in the material, in real time. AE is known to be used for monitoring the domain switching generated in ferroelectric materials under the influence of external electric fields, as reported by W. Pan and H. Cao (Ferroelectrics, 129, 119 (1992)) and Y. Saito and S. Hori (Jpn. J. Appl. Phys., 33, 5555 (1994)).
Generally, photoinduced domain switching phenomena are observed by generating and switching 180.degree. domains in ferroelectric single crystals, such as SbSI, BaTiO.sub.3 and PbTiO.sub.3, with the aid of an optical microscope. They are caused by the screening effect of photoinduced electrons on spontaneous polarization, as analyzed in reports (V. M. Fridikin, A. A. Grekov, N. A. Kosonogov and T. R. Volk, Ferroelectrics, 4, 169 (1972); A. Semenchev, V. Gavrilyatchenko and E. Fesenko, Ferroelectrcs, 157, 135 (1994)).
In ferroelectric polycrystal ceramics, on the other hand, there exist grain boundaries in which photoinduced non-equilibrial electrons can be entrapped, forming a space charge field over each grain, as reported by Land et al. (C. E. Land and P. S. Peercy, Ferroelectrics, 22, 677 (1978)). Expectedly, the space charge field is sufficient to cause the domain switching. However, since ferroelectric ceramics are composed of grains as small as several microns and the photoinduced domain switching is generated within a very shallow depth limited to the light-absorbed range, the domain switching is virtually difficult to observe with the aid of microscopes or by use of X-ray diffraction. In fact, research on the domain switching during the application of light is scarcely conducted. Related to the present invention is U.S. Pat. No. 4,344,326 with a disclosure of non-destructive testing of ferroelectric capacitors, but it is not concerned with domain switching.